Thermoplastic foam cups are widely used for serving hot and cold beverages, and large numbers of them are used daily. They generally are made from polystyrene and are readily recycled. One type of foam cup, the APPLAUSE.RTM. cup, available from Amoco Foam Products Company, has an excellent combination of insulative and aesthetic properties. The APPLAUSE.RTM. cup is a two piece cup which comprises a circumferential sidewall, an integral rolled rim at the top of the sidewall and a recessed bottom wall which is sealed to the sidewall at a point above the bottom surface of the sidewall. The APPLAUSE.RTM. cup is produced from two foam substrate pieces, called "blanks", on a 1000 F machine manufactured by Paper Machinery Corporation ("PMC") of Milwaukee, Wis. This company is widely known as a major producer of machines for manufacture of paper cups. The PMC machines are proven machines which can sustain acceptable rates of cup production for long periods.
The operation of a PMC machine to produce a paper cup from paper blanks is described in detail in U.S. Pat. No. 4,490,130, Konzal et al., issued Dec. 25, 1984. The machine on which the blanks are assembled and formed into a cup comprises a turret with a vertical axis. On the turret are a number of mandrels that project radially from the axis and are carried to each of a succession of work stations by indexing rotation of the turret. To make tapered cups, the mandrels are frustoconical in shape with each mandrel tapering in the direction away from the turret axis and having at its small diameter outer end a mandrel nose in which there are suction inlet openings. The mandrel nose is shaped to accept and hold a circular, bottom wall blank having a circumferential flange, so that the flange will not be in contact with a surrounding sidewall blank wrapped around the mandrel. In the operation of the machine, the flanged bottom wall blank is concentrically attached by suction to the mandrel nose, with its flange projecting away from the outer end of the mandrel. Thereafter, the sidewall blank is wrapped around the mandrel and the bottom wall blank. The marginal side edge portions of the sidewall blank are then overlapped and bonded to one another to form a side seam of the cup. Subsequently, it is also necessary to seal the flange of the bottom wall blank to the sidewall blank to form a bottom seam. This bottom wall sealing is performed in several processing steps, including a single heating stage using a heating gas nozzle, which are carried out at different work stations. The heating stage uses a radial flow heating nozzle, for directing a stream of heating gas, which is placed near the bottom blank for heating the sidewall. After heating the bottom blank is moved inside the sidewall blank into its final sealing position. After completion of the heating stage, the mandrel and heated cup are indexed to other work stations where the lower end of the sidewall is wrapped around and pressed against the flange portion of the bottom blank to complete the bottom sealing. Other operations may then be performed upon the top and bottom edges of the cup sidewall, such as forming the cup rim and treating the cup to increase its strength by expanding the sidewall thickness.
The manufacture of thermoplastic foam cups on the PMC machine is essentially the same operation as the paper two piece cup manufacture described in Konzal et al., with a change to a foam substrate and use of an additional second heating stage in the bottom wall sealing operation. Modifications of Konzal et al.'s process for foam cup production have been disclosed. U.S. Pat. No. 4,579,275, Peelman et al., issued Apr. 1, 1986, and incorporated herein by reference, discloses a foam sidewall blank treating process to increase the sidewall strength. A process for sidewall expansion treatment of foam two-piece cups is disclosed in U.S. Pat. No. 4,878,970, Schubert et al., issued Nov. 7, 1989, and incorporated herein by reference. However, the formation of a well bonded, leak-proof bottom seam in a two piece cup, whether foam or paper, is vital.
Bottom wall sealing problems in manufacture of foam two-piece cups on the PMC machine have occurred. Foam two-piece cup manufacture uses an additional heating stage in the bottom sealing operation; a first and a second heating stage are performed. The bottom wall sealing problems are connected to the first heating stage of the bottom wall sealing process. The first heating stage of foam cup manufacture is performed after the circular, bottom wall foam blank is installed on the mandrel nose, but before the sidewall blank is wrapped around the mandrel. After installation on the mandrel nose, the bottom blank is heated by bringing the radial flow heating nozzle described in Konzal et al. into proximity to it. After this first heating stage, the bottom wall blank on its mandrel is indexed to another work station where the sidewall blank is wrapped around the mandrel and the side seam formed, as in Konzal et al. Thereafter, the mandrel with the sidewall blank and bottom wall blank installed is indexed to another work station where the second heating stage is performed. This second heating stage in foam cup production employs the radial flow heating nozzle of Konzal et al. and is performed primarily to heat the sidewall blank. After this stage, the bottom wall blank is moved inside the surrounding sidewall blank to its sealing position along the sidewall, and sealing of the bottom wall blank to the sidewall blank is completed with the additional steps.
Applicants have found, however, that the radial flow heating nozzle described in Konzal et al., although suitable for paper cup manufacture, creates a problem which can lead to improper sealing of the foam bottom wall blank to the foam sidewall blank, resulting in an unacceptable number of leaking cups. In the first heating stage of foam cup production, the radial heating nozzle directs a stream of heating gas radially outward from the side of the heating nozzle; i.e. the gas flow direction is perpendicular to the central axis of the heating nozzle. Because the bottom wall blank is installed on the mandrel nose perpendicular to the central axis of the nozzle, the angle of incidence of the heating gas stream to the bottom wall blank is essentially zero degrees. See FIGS. 17 and 20 of Konzal et al., showing gas outlet holes 53 on the side of the nozzle (FIG. 17) and the nozzle in heating position adjacent the bottom wall blank and the heating gas flow direction (FIG. 20). In other words, the stream is aimed below the circumferential portion of the bottom wall blank which conforms to the sidewall blank. With the existing nozzle, the heating gas stream is not aimed near the fold point between the circumferential portion of the bottom wall blank and the rest of the bottom wall blank.
Further, the purpose of the prior art heating stage in paper cup production is to melt a thin thermoplastic coating on the paper blanks, which when cooled and solidified, forms the bottom wall seal between the bottom wall blank and the sidewall blanks. In paper cup production, heating to enhance shape retention of the bottom wall blank is not important, because the paper blank does not stick when being moved inside the sidewall blank. Applicants have found that a foam bottom wall blank, however, can stick to a foam sidewall blank, and heating the bottom blank to enhance its shape retention and thereby avoid sticking is highly desirable. The purpose of the first heating stage in foam cup production is therefore to "set" the bottom blank into shape. The radial flow heating nozzle directs insufficient heating gas at the bottom blank to enhance its shape retention.
Simply put, although the existing PMC nozzle can be used in producing foam cups, it was designed for paper cup production. As explained below, the prior art nozzle can produce sticking between a foam bottom wall blank and a foam sidewall blank which occurs when the bottom wall blank is moved to its sealing position, potentially leading to a bad seal.
It is the general object of this invention to provide an improved apparatus and method useful in the manufacture of two piece cups, particularly thermoplastic foam two piece cups. It is another object to provide a recessed bottom wall foam cup having an improved bottom wall seal. It is a more specific object to provide a new heating nozzle and method for the manufacture of recessed bottom wall foam cups which overcome the problem created by the prior art nozzle design. Other objects appear below. We have found the objects of the invention can be attained by the apparatus and method described below.